Where are muscarinic receptors primarily located?

Muscarinic receptors are primarily located at parasympathetic effector organs and in the brain. These receptors play a crucial role in the function of the parasympathetic nervous system, which is responsible for conserving energy and promoting "rest-and-digest" activities.

In the context of parasympathetic effector organs, muscarinic receptors can be found in various tissues such as the heart, smooth muscles, and glands. Their activation leads to a variety of physiological effects, including decreased heart rate, increased glandular secretions, and enhanced gastrointestinal motility.

Additionally, muscarinic receptors in the brain are involved in several central nervous system functions, including cognition, memory, and modulation of neurotransmitter release. This widespread distribution highlights the importance of muscarinic receptors in both peripheral and central nervous system activities, making them a key focus in pharmacological studies related to cholinergic agents.

The other options mentioned, while they may contain a few related aspects, do not comprehensively capture the primary locations of muscarinic receptors like the chosen answer does.